1. Field of the Invention
The invention relates to apparatus for the vacuum coating of webs having a three-dimensional surface structure.
The apparatus includes a vacuum chamber having two rolls for unwinding and winding the web material, a coating source disposed between the rolls in the path of the web material, and at least one reserve roll with a cooling film.
2. Discussion of Related Art
DE-AS No. 1 059 739 discloses a method in vacuum coating whereby heat-sensitive bands are passed through the coating zone on an accompanying cooling film and winding this cooling film, together with the bands in some cases to prevent the individual layers of the bands from sticking to one another. In this manner, however, coating on two sides is possible only by separating the tape and cooling film and then turning the band. The known method, however, is not sufficient when webs having an open structure in depth are involved, such as foams, nonwovens, fabrics etc., are to be coated with a relatively great amount of the coating material per unit of area.
In the case of vacuum coating by a physical deposition process such as vapor depositing and cathode sputtering, the substrate (the web of material) is subjected to great thermal stress, due to the unavoidable thermal radiation and to the heat of condensation that is released by the deposition. Since in the vacuum it is impossible to cool with gases or ambient air, the substrate is generally guided within the coating zone over a cooled surface, which is also described in DE-AS No. 1 059 739. This measure, however, promises to be successful only if the substrate has a good thermal conductivity and/or a slight thickness, and lies smoothly against the cooling surface, which is true to some extent for thin plastic films.
The circumstances are quite different in the case of materials with three-dimensional surface structure, i.e., in the case of foams, nonwovens, fabrics etc., which have pores, cavities, projecting surface particles etc., which cannot be laid smoothly against cooling surfaces, and which as a rule are poor thermal conductors and have a relatively great thickness. The coating material condenses preferentially on projecting particles of the irregular surface, where it yields its condensation heat in addition to the radiant heat. Removal of heat to the side and through the back of the substrate is negligible. The consequence is a rapid destruction of the substrate, even at low rates of deposition per unit area of the substrate. Conditions are still more unfavorable when the material is coated simultaneously from both sides, since in this case the thermal stress is doubled and no cooling can be performed in the coating zone.
Any attempt to divide the material into several batches and apply it to the web in a number of successive passes has also failed, since the roll of the material web is an ideal heat accumulator, so that the substrate temperature rose accumulatively in each pass and the substrate was destroyed in the second, or certainly at least in the third pass.